Hexamolybdenum Clusters Supported on Exfoliated h-BN Nanosheets for Photocatalytic Water Purification

Article abstract of DOI:10.1021/acs.inorgchem.0c00528

Nowadays, the development of new effective photocatalytic materials for the purification of real wastewaters and model systems containing organic molecules constitutes an important challenge. Here we present a preparation strategy for composite materials based on hexamolybdenum cluster complexes and exfoliated hexagonal boron nitride (h-BN) nanosheets. Cluster deposition on the nanosheet surface was achieved by impregnation of the matrix by a (Bu₄N)₂[{Mo₆I₈}(NO₃)₆]/acetone solution. Successful cluster immobilization and chemical composition of the samples were verified by inductively coupled plasma atomic emission spectroscopy, transmission electron microscopy with elemental mapping (TEM/EDS), X-ray photoelectron spectroscopy (XPS), and optical diffuse-reflectance spectroscopy. A small amount of water in acetone initiates the hydrolysis of a molybdenum cluster precursor with labile NO₃^- ligands, which are absent in the final composite, according to the XPS data. Intermediate hydrolyzed cluster forms anchor to the surface of h-BN nanosheets and promote growth of the insoluble compound [{Mo₆I₈}(H₂O)₂(OH)₄]·yH₂O as the final hydrolysis product. TEM/EDS proves that the cluster exists at the nanosheet surface in the form of an X-ray diffraction amorphous thin film. The samples obtained show high photocatalytic activity in the degradation of a model pollutant rhodamine B under UV- and visible-light irradiation. The materials retain their initial photocatalytic efficacy during at least six cycles without the need for recovery.

Full text of DOI:10.1021/acs.inorgchem.0c00528

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Article
Hexamolybdenum Clusters Supported on Exfoliated h‑BN
Nanosheets for Photocatalytic Water Purification
Mariia N. Ivanova,* Yuri A. Vorotnikov, Elena E. Plotnikova, Margarita V. Marchuk, Anton A. Ivanov,
Igor P. Asanov, Alphiya R. Tsygankova, Ekaterina D. Grayfer,* Vladimir E. Fedorov,
and Michael A. Shestopalov
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ABSTRACT: Nowadays, the development of new effective
photocatalytic materials for the purification of real wastewaters
and model systems containing organic molecules constitutes an
important challenge. Here we present a preparation strategy for
composite materials based on hexamolybdenum cluster complexes
and exfoliated hexagonal boron nitride (h-BN) nanosheets. Cluster
deposition on the nanosheet surface was achieved by impregnation
of the matrix by a (Bu4N)2[{Mo6I8}(NO3)6]/acetone solution.
Successful cluster immobilization and chemical composition of the
samples were verified by inductively coupled plasma atomic
emission spectroscopy, transmission electron microscopy with
elemental mapping (TEM/EDS), X-ray photoelectron spectrosco-
py (XPS), and optical diffuse-reflectance spectroscopy. A small
amount of water in acetone initiates the hydrolysis of a molybdenum cluster precursor with labile NO3− ligands, which are absent in
the final composite, according to the XPS data. Intermediate hydrolyzed cluster forms anchor to the surface of h-BN nanosheets and
promote growth of the insoluble compound [{Mo6I8}(H2O)2(OH)4]·yH2O as the final hydrolysis product. TEM/EDS proves that
the cluster exists at the nanosheet surface in the form of an X-ray diffraction amorphous thin film. The samples obtained show high
photocatalytic activity in the degradation of a model pollutant rhodamine B under UV- and visible-light irradiation. The materials
retain their initial photocatalytic efficacy during at least six cycles without the need for recovery.
INTRODUCTION
■
In the last few decades, a global problem of water pollution by
industrial products requires new effective ways of wastewater
purification. Especially, a photocatalytic approach seems to be
the most powerful and sustainable tool for the degradation of
organic pollutants, owing to its numerous advantages,
including the use of sunlight as a renewable energy source,
photocatalyst recycling without activity loss, etc. Therefore,
huge efforts have been made to develop new effective
photocatalytic materials with the required characteristics,
such as high activity, chemical stability, cost efficiency, easy
recyclability, and a range of working wavelengths close to
sunlight.1,2 Usually, TiO2-based materials were studied in this
context,3 but they have a major disadvantage of acting
effectively only under UV-light irradiation, i.e., utilizing less
than 10% of the sunlight spectrum. Many other materials have
been developed, but the task is still challenging.1−6
Figure 1. Representation of the hexamolybdenum clusters [{Mo6X8}-
L6]n. Color code: Mo, green; X, purple; L, blue.
Received: February 19, 2020
On the basis of our broad experience with octahedral
transition-metal cluster complexes, particularly, molybdenum
clusters [{Mo6X8}L6]n (X = Cl, Br, or I; L = organic or
inorganic ligands; Figure 1),7−18 we suggest considering them
as promising agents for photocatalytic water purification. Their
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controller C11924-211 (670 mW). Visible-light irradiation was
carried out by an IKEA LED lamp (110 mW; wavelength range
∼400−700 nm; Figure S1).
variable terminal ligand (L) environment provides for chemical
reactivity and solubility, while the highly stable cluster core
{Mo6X8}4+ is responsible for the photophysical properties, i.e.,
photoluminescence16−19 and the ability to generate reactive
oxygen species (ROS), such as singlet oxygen, hydroxyl radical,
superoxide radical, etc.13,14,20−23 Owing to these beneficial
characteristics, this class of compounds is already well-studied
for numerous applications, including as components of optical
waveguides,12 liquid-crystal devices,24 solar panels,25 anti-
bacterial materials,26,27 agents for bioimaging22,28 and photo-
dynamic therapy,9,10,13,14,22 and catalysts.29−31 However, the
water treatment processes employing hexamolybdenum cluster
complexes remain little explored. To the best of our
knowledge, the only two examples are the studies on
rhodamine B (RhB) photodegradation in water using a
[{Mo6Br8}(N3)6]2− cluster as a homogeneous catalyst21 or a
heterogeneous one when combined with gold nanoparticles
and graphene oxide as the support.20 However, a simple two-
component system containing a cluster and a support for
heterogeneous photocatalysis has not been described. At the
same time, cluster complexes have been often combined with
various matrixes beyond graphene oxide,20,29,30 such as MIL-
1019 and SiO2 meso- and nanoparticles,13−15,28,32−34 poly-
styrene,8,10,16,26,27,35 polyurethane,36 poly(methyl methacry-
late),12,37−39 and poly(vinylpyrrolidone).40
In our work, we propose another matrix, hexagonal boron
nitride (h-BN), to anchor cluster complexes and promote their
application in photocatalytic water purification processes. h-
BN is a layered material with a crystal structure quite similar to
that of graphite but an isolator in its physical nature, so it is
called “white graphite”. h-BN is known to have high thermal
conductivity, stability toward high temperatures and aggressive
chemical conditions (acids, bases, and oxidants), transparency
in the visible and near-UV range, etc. Furthermore, recently it
was shown that h-BN itself possesses catalytic activity in
certain processes2,41−45 and participates in photocatalysis by
separating charge carriers, e.g., holes and electrons.2,46
Therefore, h-BN nanosheets were used as supports for
photocatalytically active particles, such as anatase (TiO2),47
TiO2/Au,48,49 graphitic carbon nitride (g-C3N4),2,46 etc.
Thus, this work is aimed at the preparation and character-
ization of composite materials based on 2D h-BN nanosheets
(BNNS) with octahedral molybdenum cluster complexes and
investigation of their photocatalytic properties in the reactions
of organic dye degradation under UV- or visible-light
irradiation. Moreover, the composition, morphology, and
photocatalytic properties of the obtained materials were
comprehensively investigated.
Powder X-ray diffraction (XRD) patterns for solid samples were
collected using a Philips PW 1830/1710 automated diffractometer
(Cu Kα radiation, graphite monochromator, and silicon plate as an
external standard). High-resolution transmission electron microscopy
(HRTEM) images were obtained with a JEOL JEM-2200FS
microscope with a lattice-fringe resolution of 0.1 nm at an accelerating
voltage of 200 kV. Suspensions in ethanol were deposited on carbon-
film-coated copper grids. UV−vis absorption spectra were collected
by an Agilent Cary 60 automatic spectrophotometer.
To determine the molybdenum content in nanocomposite samples,
inductively coupled plasma atomic emission spectroscopy (ICP-AES)
analyses of solutions prepared by microwave-assisted washing of
molybdenum from a BN matrix (HF:H2O2 = 1:0.15; 150−180 °C)
were carried out on a Thermo Scientific iCAP-6500 high-resolution
spectrometer with a cyclone-type spray chamber and a “SeaSpray”
nebulizer. The spectra were obtained by axial plasma viewing.
Standard operating conditions of the ICP-AES system were as follows:
power = 1150 W, injector inner diameter = 3 mm, carrier argon flow
= 0.7 L min−1, accessorial argon flow = 0.5 L min−1, cooling argon
flow = 12 L min−1, number of parallel measurements = 3, and
integration time = 5 s. Deionized water (R ≈ 18 MΩ) was used to
prepare the sample solutions.
Optical diffuse-reflectance spectra were measured at room
temperature on a Shimadzu UV−vis−near-IR 3101 PC spectropho-
tometer equipped with an integrating sphere and reproduced in the
form of Kubelka−Munk theory. To determine the band gap (Eg) of
the samples, the Tauc plot method [(hνα)1/2 = A(hν − Eg), where h =
Planck constant, ν = frequency, α = absorption coefficient, and A =
proportionality constant] was used.
X-ray photoelectron spectroscopy (XPS) was performed on a
SPECS Phoibos-150 multi channeltron detector (MCD) spectrom-
eter with monochromatic Al Kα excitation. The electron pass energy
was 20 eV. The powder samples were pressed into double-sided
adhesive carbon tape. For neutralization of the charging effect,
irradiation of the samples by a low-energy electron beam was applied.
Calibration of the binding energies was performed relative to an
internal standard from the C 1s to 285.0 eV. Separation of the
contributions from different atoms was carried out by a fitting of
spectra on mixed Lorentzian−Gaussian symmetrical components.
Synthetic Procedures. Preparation of Exfoliated BNNS. h-BN
nanosheets were prepared by the treatment of h-BN powders with
H2O2 (30%) following a modified technique.51 Typically, 150 mg of
h-BN was put into a cylindrical Teflon autoclave (rated pressure 2−3
atm). A total of 30 mL of a H2O2 solution was added, and the mixture
was ultrasonicated for 30 min and then thermostated in a water bath
for 20 h at 80 °C. After the reaction, the solid phase of the mixture
was separated by centrifugation, washed two times with distilled water
and ethanol, and then dried at 50 °C until a constant mass was
reached.
Impregnation of BNNS with (Bu4N)2[{Mo6I8}I6]. (Bu4N)2[{Mo6I8}-
I6] (228 mg, 80 μmol) and BNNS (100 mg, 4.0 mmol) with a molar
ratio of 0.02:1 were placed in a flask, 100 mL of acetone was added,
and the mixture was ultrasonicated for 1 h. Then the precipitate was
isolated by centrifugation, washed with acetone three times, and dried
in air at 50 °C until a constant mass was reached.
Preparation of AHx/BNNS Composites by In Situ Hydrolysis of
(Bu4N)2[{Mo6I8}(NO3)6] in the Presence of BNNS. A total of 100 mg of
BNNS and 100 mL of acetone were mixed in a flask, and different
amounts of (Bu4N)2[{Mo6I8}(NO3)6] were added: 10, 50, 100, 200,
300, and 500 mg [the molar ratios of (Bu4N)2[{Mo6I8}(NO3)6]/
BNNS were 0.001:1, 0.005:1, 0.01:1, 0.02:1, 0.03:1, and 0.05:1,
respectively], which was followed by ultrasonication for 5 h. Then the
precipitate was isolated by centrifugation, washed with acetone three
times, and dried in air at 50 °C until a constant mass was reached.
The samples are referred to as AHx/BNNS (x is the moles of
(Bu4N)2[{Mo6I8}(NO3)6] on 1 mol of BNNS).
EXPERIMENTAL SECTION
■
Materials and Methods. h-BN powder (1 μm in size and
≥98.0% purity) and RhB were purchased from Sigma-Aldrich.
Molybdenum powder was annealed in a hydrogen flow and had
≥99.9% purity. (Bu4N)2[{Mo6I8}I6] and (Bu4N)2[{Mo6I8}(NO3)6]
were obtained according to previously reported procedures.16,50
Other reagents and solvents were commercially available and were
used without additional purification. Acetone contained some
moisture.
Ultrasonic treatment was carried out in a “Sapphire” ultrasonic bath
(ultrasound power 150 W; frequency 35 kHz). Centrifugation was
achieved by a Beckman Coulter Allegra X-30 centrifuge equipped with
rotor F0630 (acceleration of 20000g). UV-light irradiation was
performed by a Hamamatsu Photonics light-emitting-diode (LED)
head unit L11921-400 (wavelength 365 5 nm) used with a LED
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Preparation of Bulk [{Mo6I8}(H2O)2(OH)4]·yH2O (bAH). A total of
300 mg of (Bu4N)2[{Mo6I8}(NO3)6] was dissolved in acetone (19
mL) containing some moisture, and NH3·H2O (1.9 mL) was added
under constant stirring. The mixture was stirred for 24 h to achieve
full precipitation. The precipitate was isolated by centrifugation and
washed by acetone. The reaction yield was 186 mg, which
corresponds to 88% for y = 2.
Preparation of Nanosized [{Mo6I8}(H2O)2(OH)4]·yH2O (nAH). A
total of 300 mg of (Bu4N)2[{Mo6I8}(NO3)6] was dissolved in acetone
(100 mL) containing some moisture, and the solution was left to
stand. After 1 week, the solution had become opalescent, indicating
the formation of nanoparticles. After 2 months, a precipitate had
formed because of hydrolysis of the starting complex, was isolated by
centrifugation, washed with acetone three times, and then dried in air
at 50 °C to constant mass. The reaction yield after 2 months was 30
mg, which corresponds to 14% for y = 2.
Photocatalytic Experiments. In a typical photocatalytic test, 20 mg
of sample was placed in a quartz reactor, 80 mL of water was added,
and the mixture was ultrasonicated for 5 min. Then organic dye RhB
was added to achieve the concentration of 2.5 mg L−1. The mixture in
the reactor was vigorously stirred in darkness for 2 h to reach
adsorption−desorption equilibrium. After that, a probe (8 mL) was
collected, and the remaining solution was irradiated with UV (365
5 nm) or visible (∼400−700 nm) light. The distance between the
reactor and light source was 10 mm. At regular intervals of irradiation
time, probes were collected (5, 15, 30, 45, 60, 75, 90, 105, 120, 180,
and 300 min) and centrifuged. UV−vis spectra of the isolated
solutions were recorded in order to estimate the concentration of
RhB. The decrease of the RhB concentration was tracked by its
characteristic optical absorbance at 554 nm.
In order to verify the stability of the photocatalysts and their ability
to be used repeatedly, we carried out recycling experiments. At the
end of each run (60 min of irradiation), after the UV−vis spectrum
was recorded, the precipitate was placed back into the reaction
mixture and the initial concentration of RhB (2.5 mg/L) and volume
of the mixture (80 mL) were adjusted by the addition of a RhB
solution.
retaining the h-BN crystal structure (Figure S3, line A). H2O2
treatment functionalizes the h-BN surface with hydrophilic
−OH groups and other defects,51 rendering the product
dispersible in water (Figure S2C) and assisting the anchoring
of molybdenum clusters.
First, the BNNS sample was impregnated with an acetone
solution of (Bu4N)2[{Mo6I8}I6] under a constant ultrasonic
treatment to achieve better matrix exfoliation. The solid phase
of the mixture was thoroughly washed to remove the
unsupported cluster complex. The obtained sample had a
white color, as well as BNNS, while the initial cluster was dark-
red, thus indicating a low degree of the cluster inclusion. The
molybdenum content in the sample was less than 0.1 wt %
according to ICP-AES. Thus, impregnation of the matrix with a
(Bu4N)2[{Mo6I8}I6] solution did not lead to the anchoring of
a significant amount of the cluster. Apparently, in this system,
no notable interactions between cluster complex and BNNS
occur because of the strong bonding between the cluster core
and apical iodine ligand.
Therefore, we decided to utilize cluster complexes with
labile ligands, enabling potentially advantageous chemical
transformations. Our broad experience in cluster science has
led our attention to the hydrolysis of clusters with labile
ligands, producing an insoluble aqua−hydroxo complex,
[{Mo6I8}(H2O)2(OH)4]·yH2O (AH).7,9,14,52−54 Therefore, to
fabricate the h-BN/cluster nanocomposites, we designed and
experimentally tested a special one-pot process featuring the in
situ hydrolysis of (Bu4N)2[{Mo6I8}(NO3)6] with a labile
NO3− ligand during BNNS impregnation. Taking into account
the synthetic conditions, we can suggest that the parent cluster
transforms into insoluble AH. By variation of the initial
mixture composition, a series of samples AHx/BNNS were
obtained (x is the moles of the cluster (Bu4N)2[{Mo6I8}-
(NO3)6] on 1 mol of BNNS). The orange color of the
resulting samples indicates the successful inclusion of a cluster
complex. The molybdenum content found by ICP-AES in the
composite depends on the initial component ratio in the
synthetic mixture, as illustrated in Figure 2.
To shed light on the mechanism of photocatalysis, scavengers for
ROS and other active species were used: ammonium oxalate for holes,
ascorbic acid for superoxide anion radicals and singlet oxygen, and
ethanol for hydroxyl radicals. A 1 mmol L−1 scavenger was added to
the reaction mixture during the typical photocatalytic experiment, and
its effect on the RhB degradation was monitored.
Up to x = 0.03, the molybdenum content in the samples
rises linearly with an increase of the cluster concentration in
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Materials. Octahe-
dral molybdenum cluster complexes possess interesting
properties, particularly, ROS generation.13,14,20−23 This prop-
erty can be used to develop new cluster-based photocatalysts
for the removal of organic pollutants from wastewater.
However, to stabilize the catalytically active centers, facilitate
the separation of the material used, and improve its functional
characteristics, it is of practical interest to develop new hybrid
or composite materials with molybdenum clusters and an
appropriate matrix. h-BN seems to be very perspective in such
a role because of its high thermal and chemical stability,
transparency in the visible and near-UV range, and ability to
effectively separate charge carriers, preventing their recombi-
nation. However, h-BN in its original form is a chemically
inert, hydrophobic, and low-surface-area material. For use in
composites, these issues may be successfully overcome by
surface functionalization, exfoliation, and the addition of
potential anchoring sites. Therefore, for cluster immobilization,
we used h-BN pretreated by H2O2 in an autoclave at 80 °C
following our previously developed method.51 This procedure
gives exfoliated BNNS with thicknesses of up to several layers
and average lateral sizes of 200−600 nm (Figure S2A,B,D),
Figure 2. ICP-AES data on the molybdenum content in the AHx/
BNNS versus component ratio (x is the moles of (Bu4N)2[{Mo6I8}-
(NO3)6] to 1 mol of BNNS). The line is an approximation of the data
(for x from 0.005 to 0.03).
C
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Figure 3. Simplified process of (Bu4N)2[{Mo6I8}(NO3)6] hydrolysis in water-containing media described as parallel steps: (1) substitution of
−
−
NO3 ligands with water molecules (the release of free NO3 groups should be taken into account); (2) removal of protons from coordinated
water, resulting in coordinated OH−; (3) transformation of soluble forms [reactions (1) and (2)] into the insoluble AH compound.
Figure 4. TEM/HRTEM images of the samples BNNS (A and D), AH0.01/BNNS (B and E), and AH0.05/BNNS (C and F).
the initial mixture. However, at x = 0.05, a sharp decrease in
the molybdenum content is observed, which indicates a lower
yield of the insoluble AH. This effect is well reproducible and
seems to be due to both kinetic and thermodynamic reasons.
Hydrolysis of (Bu4N)2[{Mo6I8}(NO3)6] in solution is a
complicated and pH-dependent multistep process (Figure
an increase of the DMSO concentration in solution resulted in
a decrease of the hydrolysis rate.
Additional data on the composition as well as on the
morphology of the samples were obtained by TEM/EDS
(Figure 4). In the case of samples with clusters, the observed
particle sizes of around several hundred nanometers (Figure
4B,C) well correspond to the size of BNNS (Figure 4A, D). It
is clearly seen in HRTEM images (Figure 4E,F) that the
BNNS surface is decorated by a thin film, containing Mo and I
atoms, as proven by elemental mapping (Figures S4C,F,G and
S5C,F,G). This film is probably amorphous because powder
XRD patterns show only reflexes from well-crystalline h-BN
even for the highest molybdenum content (Figure S3, lines B
and C). In the AH0.01/BNNS composite, there are areas of free
h-BN surface because the molybdenum content is relatively
9
−
3). It starts with the substitution of NO3 ligands with water
molecules (reaction 1), followed by water deprotonation
(reaction 2), which can occur in parallel. It seems that the first
stages of the hydrolysis (1) and (2) in Figure 3 proceed fast
and produce many soluble partially substituted forms. After
some time, various forms can be found in the solution
experimentally,9 which ultimately transform into nonsoluble
AH ([{Mo6I8}(H2O)2(OH)4]·yH2O) [reaction (3)]. At high
(Bu4N)2[{Mo6I8}(NO3)6] concentration (x = 0.05), the final
steps of hydrolysis (3) are inhibited. To explain this
phenomenon, we can put forward two assumptions. First, it
can be due to a lowered water concentration in the mixture. In
our experiments, a water admixture, which is essential for
hydrolysis processes, was being wasted as a ligand in reaction
(1) and coprecipitated with AH in reaction (3). Second, AH
low (Figures 4B,E and S4). On the other hand, in AH0.05
/
BNNS, when the molybdenum content is higher, most of the
BNNS surface is covered by a cluster-containing amorphous
phase (Figures 4C,F and S5).
To shed light on the possible mechanism and to obtain a
clue to the role of the colloidal BNNS support in the formation
and attachment of the AH compound, we prepared an
unsupported AH cluster complex under conditions very close
to those used for composites AHx/BNNS (Figure S6). The
TEM images show that the sample contains particles of 30−50
nm size (nanosized sample referred to as nAH), which are
amorphous according to the electron diffraction image (Figure
S6B) and XRD (Figure S7). The AH phase on the surface of
−
precipitation leads to an increase in the free NO3
concentration relative to the total cluster complex concen-
tration in solution, thereby shifting the equilibrium of reaction
(1) to the left. A similar behavior was shown in our previous
work7 on a related cluster complex [{Mo6I8}(DMSO)6]4+with
another labile ligand, namely, dimethyl sulfoxide (DMSO); i.e.,
D
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Figure 5. XPS spectra showing the N 1s (A), B 1s (B), Mo 3d (C), and I 3d (D) core levels in the AH0.05/BNNS composite.
BNNS is also amorphous but exists in the form of a continuous
thin film (Figure 4E,F). Different morphologies of free and
deposited AH phases indicate the important role of the BNNS
support in AH formation. Overall, AH growth in the presence
of the hydrophilic support BNNS preferably in the form of an
extended thin film may proceed around active centers.
Probably, partially hydrolyzed cluster forms coordinate to
surface edges, defects, and functional groups.
3d3/2 and Mo 3d5/2, additional low-intensity doublet (peaks at
235.6 and 232.4 eV) was observed (Figure 5C). These peaks
are probably due to slight cluster oxidation to the MoVI state
under ultrasonication because in nonsonicated bAH only MoII
atoms were found (Figure S10). Also, in the AHx/BNNS
spectra, only N atoms related to h-BN were observed (Figure
−
5A), indicating the complete removal of NO3 groups during
in situ hydrolysis of the cluster precursor. Therefore, XPS does
not reveal any specific interactions between the cluster
complex and h-BN, suggesting deposition without chemical
bond formation.
AHx/BNNS and reference samples [the initial BNNS and
unsupported bulk AH (bAH)] were studied using XPS. The
spectrum of the initial BNNS shows characteristic boron (B 1s,
190.84 eV) and nitrogen (N 1s, 398.74 eV) peaks and a small
amount of surface oxygen (Figure S8). After cluster deposition,
the BNNS composition remains unchanged; however, as
expected, the survey spectrum shows the appearance of new
cluster-related peaks, namely, Mo 3d3/2 (232.5 eV), Mo 3d5/2
(229.2 eV), I 3d3/2 (632.5 eV), and I 3d5/2 (621 eV), which
proves composite formation (typical spectra of the composites
are shown in Figures 5 and S9). The molybdenum and iodine
peak positions correlate perfectly with literature data for
octahedral molybdenum iodide clusters and cluster-containing
materials.55,56 Figure 5D demonstrates two additional
shoulders at lower binding energies (619.5 and 630.8 eV)
near the main peaks of the inner iodine ligands (Ii), which
correspond to some unreacted apical ligands (Ia). This fact
indicates that not all apical iodine ligands were replaced by
Cluster complex deposition on the surface of BNNS leads to
a change in the sample color to orange; therefore, we decided
to investigate their optical properties. The recorded diffuse-
reflectance spectra of AHx/BNNS composites and both
individual compounds (BNNS and bAH) were converted to
absorption spectra using the Kubelka−Munk function (Figure
S11A). Data obtained show that individual BNNS have no
absorption in the studied region (250−800 nm). For
composite materials, an absorption band is observed up to
700 nm. It should be noted that the profiles of the absorption
spectra of the nanocomposite materials and bAH are similar,
which further confirms cluster complex deposition on the
BNNS surface in the AH form. To measure the band gap (Eg)
of the materials, the Tauc plot method was used (Figure
S11B). The Eg value of the initial BNNS lies outside the limits
of the studied region and exceeds 5 eV.57 The Eg value for bAH
−
NO3 during the cluster precursor preparation. Regarding the
region of the Mo 3d core level, besides the main doublet of Mo
was estimated to be ∼1.7 eV. For the composites AH0.03/
E
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Figure 6. (A) C/C0 ratio versus irradiation time for the photocatalytic decomposition of RhB under UV light (365 5 nm): blank experiment
without any catalyst; photocatalytic degradation on the bAH, BNNS, nAH, and AH0.03/BNNS (top-down). (B) UV−vis spectra of a RhB solution
before and after irradiation during different time intervals (up to 120 min) in the presence of AH0.03/BNNS. The insets are structures of RhB and
intermediate degradation product (deethylated form) (up) and solution photographs (down). (C) C/C0 versus irradiation time for the
photocatalytic decomposition of RhB in the presence of AH0.03/BNNS under visible light. (D) UV−vis spectra of a RhB solution before and after
irradiation during different time intervals (up to 300 min) in the presence of AH0.03/BNNS. Gray areas show the 2 h sorption stages.
BNNS and AH0.05/BNNS, the values were close to that of the
unsupported bAH. Overall, cluster deposition results in the
appearance of absorption in the visible region, leading us to
study the AHx/BNNS photocatalytic properties.
composites appears to be caused by the coaction of both
components, and BNNS acts not simply as a carrier but also as
a cocatalyst.2,41−45 Along with a decrease in dye absorption,
bands related to the consecutive formation of deethylated RhB
forms appear at 500−540 nm (Figure 6B). RhB deethylation is
typical for photocatalysts based on molybdenum clusters20,21
and many other materials.2,59,60 When irradiation is prolonged,
these bands also finally lose their intensity, indicating further
decomposition (Figure 6B).
The best activity was demonstrated by AH0.03/BNNS
(Figures S13 and S14) probably because of the highest
amount of cluster and, therefore, increased coating of BNNS
by the AH film. The value of the reaction rate constant
(pseudo-first-order reaction) for AH0.03/BNNS was estimated
as 0.013 min−1 (Figure S15). It is worth noting that the
reaction rate depends on many parameters, including the dye
and catalyst concentrations and lamp power, so a comparison
with other catalysts is rather qualitative, especially taking into
account that this work is one of a few studying the
photocatalytic dye decomposition on hexamolybdenum
clusters.20,21 Overall, the obtained constant value is close to
those of other promising heterogeneous photocatalysts in the
RhB degradation process.2,61
Photocatalytic Properties of Materials. The AHx/
BNNS samples were tested as photocatalytic materials in the
degradation of RhB as a model organic pollutant (Figure 6).
Because the aqueous solution of RhB has an absorption
maximum at 554 nm, a decrease in the intensity of this
absorption band was used to monitor the decomposition
process. RhB is quite stable under UV-light irradiation (the
decomposition degree is less than 10% after 120 min; Figure 6
A). Before the photodegradation study, a mixture of dye and
photocatalyst was stirred for 2 h in the dark to achieve
sorption−desorption equilibrium (Figure S12).58 Figure 6A
shows the values of C/C0 of RhB after the sorption stage (0
min of irradiation) and after different time intervals of
irradiation of BNNS and various samples: bAH, nAH, and
composite AH0.03/BNNS. In the cases of individual compo-
nents (BNNS, bAH, and nAH), decomposition proceeds much
better than that in the blank experiment without any catalyst
(Figure 6 A), but the reaction was not completed even in 120
min. It should be noted that the nanosized form of AH (nAH)
has better photocatalytic properties than the bulk one (bAH).
At the same time, the composites AHx/BNNS show the best
properties, making the RhB solution colorless after UV-light
irradiation for 60−90 min. The better performance of the
The prolonged activity and stability of the materials AH0.03
/
BNNS were investigated by cycling experiments (Figures 7 and
S16). As was mentioned above, complete RhB degradation
with AH0.03/BNNS requires 60 min of irradiation. After each
F
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state quantum yield. Recombination effects may be somewhat
diminished by the h-BN support because it favors charge
separation.2,46,62 Surface holes can also react with hydroxide
ions to form a highly active hydroxyl radical (OH•), while
surface electrons convert molecular oxygen dissolved in the
system into superoxide radicals (O2•−). In addition,
molybdenum cluster complexes are well-known for their
efficiency in the generation of singlet oxygen (1O2).11,13,14,23
All of these highly reactive species can oxidize dye molecules
dissolved in water, leading to their destruction and ultimate
mineralization of the mixture.20,21,46
To gain insight into the role of different potential active
species in our photocatalytic process, we conducted trapping
63
•−
1
experiments with ascorbic acid (O2 and O2 scavenger),
ethanol (OH• scavenger),64 and ammonium oxalate (h+
scavenger).65 A photocatalytic activity drop in the presence
of a certain scavenger indicates that the target species has an
active part in the process.4−6 As follows from Figure 9, only
ascorbic acid is capable of preventing complete RhB
degradation, thus indicating that O2•−/1O2 as probable
photocatalytically active species.
Figure 7. Cycling of AH0.03/BNNS in RhB photocatalytic
decomposition.
run, the initial concentration of RhB and the volume of the
mixture were readjusted. The resulting material retains its
efficiency at least for six cycles. The good stability during the
photocatalytic reactions is probably related to several factors,
such as (i) insolubility of the AH cluster, (ii) effective
stabilization of the cluster phase on surface defects or/and
functional groups, and (iii) photocatalytic mechanism, which
does not include active site dissolution (see below).
As shown earlier, cluster deposition led to an increase in
AHx/BNNS light absorption in the visible region; therefore, we
further studied the photocatalytic properties utilizing an IKEA
table lamp, emitting in the range of ∼400−700 nm (Figure
S1). RhB completely decomposes in the presence of a AH0.03
/
BNNS composite, although it takes somewhat longer than
under UV irradiation (Figure 6C,D). The degradation
proceeds through the formation of intermediate deethylated
forms, similar to the case of UV irradiation. This experiment is
very important for further practical applications in sunlight-
driven photocatalytic water purification. Because the sunlight
spectrum contains both UV and visible components, a higher
efficiency of the catalysts is expected.
Study of the Photocatalytic Mechanism. Apparently,
RhB decomposition is a complex photocatalytic process that
causes nonideal shapes of the curves (Figure 6). The overall
mechanism may be imagined as schematically drawn in Figure
8. The absorption of a light quantum leads to the transition of
the cluster fragment to the long-lived excited state, producing
an electron−hole pair. Both active species can be separated by
diffusion to the AH film surface; however, their unfavorable
recombination may occur, leading to a decrease in the excited-
Figure 9. C/C0 ratio versus irradiation time for photocatalytic RhB
decomposition on AH0.03/BNNS under UV light: without any
scavenger and in the presence of ascorbic acid, ethanol, and
ammonium oxalate. The gray area shows the 2 h sorption stage.
Although superoxide radical formation by [{Mo6•X−8}L6]
cluster complexes is unfavorable,23 we suppose that O2 may
still be involved in the photocatalytic process because h-BN is
2,66
able to stabilize electrons2,46,62 and generate O2 itself.
At
•−
11,13,14,23
1
the same time, cluster complexes produce O2 well.
These observations once again indicate that both components
of the AH/BNNS catalyst participate in the photocatalytic
process.
CONCLUSION
■
The present study demonstrates the deposition of hexamo-
lybdenum cluster complexes onto hydrophilic functionalized h-
BN nanosheets. The key to a successful immobilization was the
use of a soluble precursor cluster with labile apical ligands
(Bu4N)2[{Mo6I8}(NO3)6] able to undergo in situ hydrolysis
resulting in formation of the insoluble [{Mo6I8}-
(H2O)2(OH)4]·yH2O on a modified h-BN nanosheet surface
as a thin amorphous film. The reaction of RhB degradation
modeling wastewater purification was used to demonstrate the
Figure 8. Schematic of the photocatalytic process of organic dye
decomposition in the presence of composite AH/BNNS. AH is
[{Mo6I8}(H2O)2(OH)4]·yH2O.
G
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practical potential of the designed catalyst. In contrast to
individual h-BN nanosheets and cluster complexes, nano-
composites exhibited full decomposition of the dye under both
UV and visible light, retaining their activity for at least six
cycles. Scavenger experiments reveal the important contribu-
tion of the O2•− and 1O2 to the photocatalytic process. A better
performance of composite materials appears to be caused by
the coaction of both components: (i) cluster complexes have
high absorption in the visible region and noticeable photo-
catalytic properties; (ii) h-BN stabilizes the active phase in
thin-film morphology, possibly enhancing dye sorption,
benefits effective charge-carrier separation, and even manifests
its own additional catalytic activity. Overall, this study
evidences that molybdenum cluster complexes are promising
candidates to obtain nanocomposites with advanced character-
istics and opens up rich possibilities for future research
directed at the catalytic elimination of pollutants, ecofriendly
catalysts, and other areas related to environmental protection.
Vladimir E. Fedorov − Nikolaev Institute of Inorganic
Chemistry, Siberian Branch of the Russian Academy of Sciences,
630090 Novosibirsk, Russian Federation
Michael A. Shestopalov − Nikolaev Institute of Inorganic
Chemistry, Siberian Branch of the Russian Academy of Sciences,
630090 Novosibirsk, Russian Federation; orcid.org/0000-
0001-9833-6060
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00528
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was supported by the Russian Foundation for Basic
Research (Grant 17-03-00140). E.D.G. and M.N.I. thank the
President of the Russian Federation for scholarship awards
(CΠ-140.2018.1 and CΠ-3729.2019.1, respectively).
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00528.
LED lamp spectrum, HRTEM, EDS, XRD, XPS, diffuse-
reflectance spectra, sorption of RhB, UV−vis spectra of
RhB solutions, kinetic study, and cycling experiments
(PDF)
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AUTHOR INFORMATION
Corresponding Authors
■
Mariia N. Ivanova − Nikolaev Institute of Inorganic Chemistry,
Siberian Branch of the Russian Academy of Sciences, 630090
Novosibirsk, Russian Federation; orcid.org/0000-0001-
5798-7211; Phone: +73833309253; Email: kozlova@
niic.nsc.ru; Fax: +73833309489
Ekaterina D. Grayfer − Nikolaev Institute of Inorganic
Chemistry, Siberian Branch of the Russian Academy of Sciences,
630090 Novosibirsk, Russian Federation; orcid.org/0000-
0002-2994-959X; Phone: +73833309253; Email: grayfer@
niic.nsc.ru; Fax: +73833309489
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Elena E. Plotnikova − Nikolaev Institute of Inorganic
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Margarita V. Marchuk − Nikolaev Institute of Inorganic
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Anton A. Ivanov − Nikolaev Institute of Inorganic Chemistry,
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2026-8568
Igor P. Asanov − Nikolaev Institute of Inorganic Chemistry,
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Alphiya R. Tsygankova − Nikolaev Institute of Inorganic
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